The present invention relates to an assembly and process for applying a mixture of chemical components, for producing polyurethane foam, onto a longitudinally advancing metal strip. The invention finds application in the production of foam-insulated panels used in overhead doors, insulated building panels and insulating board.
It is conventional to produce foam-insulated panels as follows:
a pair of metal strips, one spaced directly above the other, are advanced together,
a mixing/dispensing assembly is positioned between the strips. The assembly comprises a polyurethane mixing head having an attached dispenser, for admixing the polymerizable chemical components (organic resin and polyisocyanate) which react to produce polyurethane foam and delivering the produced mixture onto the lower strip;
the mixing/dispensing assembly is reciprocated transversely across the advancing lower strip to distribute the mixture thereacross; and
the strips then converge closer together and are advanced through a long heating oven to cause the mixture to foam, fill the space between the strips and solidify into insulating polyurethane foam which is bonded to the two strips.
There are a number of commercially available mixing heads on the market for mixing the chemical components. A typical example brings separate streams of the two polyurethane foam-producing components through a pair of passageways formed in the mixing head and discharges them under pressure through opposed outlets into a mixing chamber and an angularly positioned port leading to the dispenser. The components mix turbulently as they meet and proceed together through the chamber and port.
There are also a number of commercially available dispensers, each of which is mounted on a reciprocating arm or carriage, for distributing the foam mixture produced by the mixing head onto the moving strip. For example, one known dispenser is a tube, perforated along its length and closed at its outer end, which delivers the mixture in the form of a plurality of parallel streams of small diameter. A second known dispenser is a hopper-like nozzle having a bottom slot which delivers the mixture in the form of an elongate, thin curtain.
In connection with these assemblies it is desirable:
that the components be well mixed;
that the mixture be evenly distributed over the bottom strip; and
that the assembly not plug readily, so that it can operate for several hours without interruption.
However, in our experience these objectives have not been achieved using the commercial mixing/dispensing assemblies heretofore available. In our testing with a variety of mixing heads and dispensers, of conventional design and of our own design, we repeatedly experienced rapid plugging, often within a few minutes. The chemical mixture is very sticky and tends to adhere to the metal surface of the mixing head port and to the internal surfaces of the dispenser. The adhering material will foam up and plug one or both of these devices. In addition, even distribution of the mixture was difficult to sustain. For example, in the case of the perforated tube, some of the perforations would begin to plug while others would remain open - as a result distribution would become uneven. There would be areas where less than the optimum quantity of mixture would be deposited. These plugging and distribution problems led to one or more of the following undesirable results:
Void spaces being present in the cured insulation;
Poor bonding of the cured insulation to the metal strips;
Costly shut-downs to deal with the plugging; and
The production of a high proportion of poor quality or defective panels, with corresponding losses and customer dissatisfaction.
In accordance with the present invention, a spinning cone, having a downwardly sloped surface, is positioned in spaced relation directly beneath the port outlet from the mixing head. The head and spinning cone are reciprocated together across the advancing bottom strip to distribute the chemical mixture along a zigzag path. Preferably the adjacent strips of laid-down mixture overlap along their margins.
It has been shown that the spinning cone is not susceptible to plugging. In addition it appears to contribute to improved mixing of the chemical components.
In a preferred feature, the cylindrical surface of the port of the mixing head is coated with a non-adhering material such as Teflon(trademark). As a result of this change, plugging of the port has been reduced. It has also been found useful to coat the top surface of the cone with Teflon or the likexe2x80x94the build-up of foam thereon is thereby reduced.
By combining these features, test runs of 5-8 hours duration have been achieved without plugging. In addition, improved mixing of the mixture components and uniformity of distribution of the mixture has been achieved, thereby reducing the incidence of void spaces and improving bonding of the foam to the metal skin.
Broadly stated, in one aspect the invention comprises a method of applying a liquid mixture of polyurethane foam components onto the top surface of a metal strip being advanced longitudinally, comprising: delivering a stream of the mixture from a mixing head having an outlet onto the apex of a spinning cone positioned directly beneath the outlet and above the strip, to dispense the mixture onto the strip; and reciprocating the mixing head and cone together transversely across the longitudinally advancing strip to evenly distribute the dispensed mixture across the top surface of the strip.
In another aspect, the invention comprises a reciprocating mixing/dispensing assembly for delivering a mixture of polyurethane foam components onto a longitudinally advancing strip, comprising: a mixing head for mixing the components and producing a mixture thereof through an outlet; a cone, having an apex positioned centrally beneath the outlet; means for spinning the cone to dispense the mixture onto the strip; and means for reciprocating the mixing head and cone together across the strip to distribute the dispensed mixture thereon.